Plastic extrusion processes usually produce different contaminants like smoke, solid particles, vapours and plate off. Those contaminants are a major factor in polluting the production environment. Some of them are a direct health hazard while others will pollute the product and affect the quality and composition of the final product. Thus, periodic cleaning of the sensitive equipment employed in plastics extrusion processes is required to maintain a clean production environment.
Usually such cleaning tasks are performed manually and must be repeated according the production volume and the sensitivity of the product and the production equipment. This requires periodic production shut-down, which is economically undesirable.
Ventilation of the production area is one of the methods used to reduce the pollution impact. However, ventilation and the resulting quick air drafts have inherent problems that may effect the production process so that ventilation by no means is an effective solution to air borne contaminants. The effectiveness of the ventilation is also dependent from vent location and even weather factors. Moreover, ventilation is an expensive pollution control method, since large amounts of air need to be replaced and conditioned to comfort level for the operators and the correct ambient temperature for the production process. Thus, a more effective and efficient pollution control process is desired.
Plastics are generally extended into solid parts or film. Plastics films are usually blown. All blown film is extruded either vertically, up or down, or horizontally. In all instances, as the melted polymeric material exits the cylindrical die as a tube, air is passed through the die into the tube to inflate the tube to form a tubular bubble, the passage of air being controlled to provide the bubble with a desired diameter. In addition, as the tube leaves the die, it is cooled by air blown from an annular nozzle or nozzles provided in an air cooling ring, a so called “air-ring”. An internal bubble cooling system may also be present by which air is continuously exchanged inside the bubble to cool the bubble from the inside.
The ring is connected to an air plenum chamber which supplies large quantities of air to the outside of the bubble so that the film material becomes firm before it is passed between two rollers (usually known as “nip rollers”) of a tube collapsing system downstream from the die. A typical prior art air ring is shown, for example, in U.S. Pat. No. 4,750,874, issued Jun. 14, 1998 to Keim.
During the film blowing process, the blow up ratio and rate of change in film thickness of the tubular bubble are at least partly dependent upon the flow rate of cooling air directed onto the tubular bubble immediately after it leaves the die orifice. To adjust these parameters one can adjust the flow rate of cooling air through an annular nozzle which lies closely adjacent to the die orifice. Adjustment of the cooling air flow rate is known to be a fine tuning operation to produce a required blow up ratio and film thickness suitable for a particular polymer. It is also known that the cooling air film must be as equal and evenly distributed as possible along the circumference of the bubble in order to avoid localized distortion or thickness variations in the film.
Molten polymeric material, upon issue from an extrusion die orifice, is accompanied by undesirable contaminants, such as smoke, odorous fumes and other airborne contaminants resulting from the extrusion process. Processing aids for the improved processing of the materials extruded also contribute to contamination of the working environment. Processing aids generally are no longer useful after they are heated and exit the die. All of these contaminants serve to increase pollution of the atmosphere immediately within the working environment adjacent to the extrusion apparatus and progressively pass into and pollute the surrounding atmosphere within a factory. Hence, such contaminants present an uncomfortable and possibly unhealthy atmosphere in which to work and may lead to pollution of the product. It would be desirous, therefore, if some means were to be found for at least reducing contaminant infiltration into the atmosphere.
U.S. 2005 0285315 (published Dec. 29, 2005) describes a blown film manufacturing process wherein the cooling air used to cool the exterior of the bubble is drawn off by way of a suction ring immediately upstream of the nip rollers.
EP 0950499 (published Oct. 20, 1999) discloses a method for manufacturing a blown film, wherein a laminar air flow screen, in addition to the cooling air flow generated by the air ring, is used to prevent dust particles from coming into contact with the film bubble.
Canadian Patent Application 2,315,463 discloses a system for evacuating contaminants in a conventional air ring setup. However, this application discloses a vacuum system placed directly against the air ring and at the extrusion nozzle. It is even suggested that the evacuation system inlet be placed adjacent to or replace the primary air outlet of the air ring. It has now been discovered that this placement of the evacuation system intake is disadvantageous. Interference with the air flow for the air ring may significantly impact the performance of the air ring and may lead to unpredictable variations in the fluidness and of the extruded film and deformation of the extruded film bubble.